Recent types of optical fibers have generally been produced from quartz glass and have widely been used as light guides for transmitting information. However, quartz glass cannot transmit light rays whose wavelength is longer than about 2 .mu.m, and, therefore, quartz glass fibers cannot be used as an infrared light guide. Infrared light transmitting optical fibers are expected to be used as fibers for connecting infrared light to a remote photodetector, e.g., an infrared light alarm or optical pyrometer. Alternatively, the fibers are used for directing high-power infrared light such as CO laser beams or CO.sub.2 laser beams to a remote or otherwise inaccessible site for use, e.g., as a laser scalpel, laser heater, or laser working machine. For such various applications, demand for fiber that can transmit infrared light has recently increased. Most of the materials that transmit infrared light are crystalline, and such crystalline fibers can be produced from metal halide crystals such as silver halide and thallium halide crystals. However, such crystal fibers have been conventionally produced by methods other than drawing through a die. For example, N. S. Kapany, "Fiber Optics Principle and Applications" (Academic Press, New York, 1967), Chapter II Infrared Optics reports on drawing of infrared transmitting optical fibers, but this prior art reference does not describe drawing of crystalline materials through a die into a fiber.
A fiber is produced from these crystals by plastic working, and to minimize the resistance to deformation, hot extrusion is conventionally utilized as a technique of plastic working. However, if a fiber is to be produced from an extrusion billet of large size by a single step of extrusion, the necessary degree of working is increased and the fiber formed often tends to have a rough worked surface. Such a rough worked surface results in light scattering at surface defects in the fiber, which increases the loss in light transmission and/or decreases the mechanical strength of the fiber. These phenomena are factors that greatly impair the characteristics of the fiber.
Heretofore, a step-index type infrared light transmitting optical fiber has been produced by inserting a cylindrical core crystal into a hollow cylinder of cladding crystal to form an extrusion billet, which is then hot-extruded into a fiber. One defect of such a fiber is that, as shown in FIG. 1, the core crystal 11 and the cladding crystal layer 12 intermesh with each other since they are deformed during extrusion, and in consequence, a rough core-cladding interface is produced that increases the scattering of the light being transmitted.